Sen. McCaskill will spend the next three weeks in Missouri undergoing treatment. For any Senate votes while she's away, she will post on her website how she would've voted and will have that information entered into the Congressional Record.

According to Susan G. Komen for the Cure, breast cancer is the most common cancer among women, and it's estimated nearly 250,000 cases of invasive breast cancer will be diagnosed in the U.S. in 2016.